Tubular electric heating element and method of making the same



Dec. 6, 1949 4 Q B, BACKER 2,490,611

TUBULAR ELECTRI EATING ELEMENT AND METHOD OF KING THE SAME Filed April 18, 1946 Patented Dec. 6, 1949 TUBULAR ELECTRIC HEATING ELEMENT AND METHOD OF MAKING THE SAME Christian B. Backer, Ottawa, Ontario, Canada Application April 18, 1946, Serial No. 663,201 In Canada May 12, 1945 6 Claims. l

This invention relates to tubular electric heating elements and method of producing the same.

The invention has particular relation to a. common type of electric heating element which is now in widespread use and which comprises an cuter metallic tube or sheath, a centrally located resistance wire coil within the sheath, and electric insulation between the coil and sheath, the electric insulation consisting of crystalline magnesium oxide produced in situ.

The procedure heretofore followed in the production oi these elements is to insert into the outer tube or sheath a magnesium metal lining, which may consist of a wound up magnesium ribbon, or of several longitudinal magnesium strips bent into segmental form, or of a seamless magnesium tube. The resistance wire coil is inserted in the center of this assembly and held in place by a suitable clip at each end of the tube.

The magnesium is then converted into magnesium hydroxide by treatment in an autoclave with steam or water at high pressure and temperature. It will be realized that the component parts of the assembly must be so arranged that the water or steam may ow through the tube and come into contact with the magnesium metal to be converted. The hydrated magnesium (niagnesium hydroxide) occupies substantially twice the space originally filled by the magnesium liner. Thus as hydration proceeds the liner grows in volume contacting the resistance coil and portions of it are forced between the convolutions of the coil. Usually substantially one half of the circumferential surface of the Wire is surrounded by the magnesium hydroxide and the surface of the portions of the latter exposed between the convolutions of the coil progressively become so hard and dense that hydration is greatly retarded. Accordingly, while much of the magnesium of the liner is hydrated in a short time after the pressure in the autoclave reaches 300 to 400 pounds per square inch, hydration of the remaining portion requires a pressure of about 1000 pounds per square inch for a period of several hours. the pressure of the coil makes it obligatory for portions of the expanding liner to be densely compacted within the narrow spaces between the convolutions of the coil leaving a greatly restricted surface area of dense material through This is due to the fact that which the hydrating agent must penetrate to complete the hydration of the magnesium of the liner. The hydrating operation thus consumes 8 to 10 hours and sometimes longer and requires high pressure for much of that time to insure effective hydration. This is an expensive and time-consuming opera-tion.

Furthermore insertion of the resistance coil prior to hydration makes it difficult to provide a cool terminal for the element. In order to permit passage of the hydrating medium into and throughout the element to hydrate the magnesium of the liner the end of the resistance wire is brought outside the element for subsequent attachment of the terminal member. If the terminal member were attached to the resistance wire at a point within the small tubular element it would interfere with admission of the hydrating medium. The resistance wire extending to the end of the element causes generation of undesirable heat in the end portions of the element, giving rise to hot terminals. The only step that can be taken towards attaining cool terminals is to run the resistance wire straight (instead of coiled up) for a suitable distance near both ends of the element tube, but even a straight resistance wire generates considerable heat. To x the resistance coil to a solid and fairly heavy terminal member, which may extend several inches into the tube, is impossible, because it would block up the passage of water through the tube during conversion. The problem of attaining cool (or reasonably cool) terminals has therefore caused great manufacturing diiiculties in the production of this type of tubular element.

It is an cbj ect of the present invention to provide a method of producing a tubular electric heating element of the type described wherein the step of converting the magnesium metal into 5 hydroxide is effectively and completely accomplished in a rapid and inexpensive manner and whereby the association of a satisfactory termiminal construction is rendered convenient and free from 'manufacturing difficulties.

The invention contemplates the provision of a magnesium hydroxide lining in a sheath prior to, instead of after, the insertion of the resistance coil into the sheath, it having been found that when the coil, which interferes greatly with the conversion process, is not present, the conversion treatment proceeds much more rapidly and at a reduced pressure. It has also been found that there may be produced a completely satisfactory element wherein the resistance coil nts snugly within the hydroxide lining but is not embedded therein during hydration.

The invention will be described with reference to the accompanying drawing, in which Figure l is an enlarged sectional elevation of an end portion of a tubular element in accordance with the invention, and

Figure 2 is a transverse sectional view.

In accordancewith the invention, a preferably seamless `magnesium metal tube is inserted into a metal sheath I. The assembled sheath and tube are then placed in an autoclave and the magnesium tube converted tofthe hydroxide with steam or water. Since the central opening in the magnesium tube is wide open and completely unobstructed, the water or steamV passing through this opening is free to contact the whole surface of the liner and the expanding liner isnot subjected to compacting pressure. Conversion of the metal therefore proceeds rapidly and may be completed in from two to four hours at 290 to 5'0'0 pounds pressure atthe -corresponding Ytemperature vvforfsaturatedPsteam,'instead of from eight l toten hours -at "1-000 pounds pressure when the tube opening 4is obstructed bythe resistance coiLa-s in the usual method. As the tube comes from the autoclave a `lining 2 is substantially completely hydrated, -is 'of luniforrn density throughout its length-iandfhas a smooth circular bore into which vthe're'sistance'coil may now be inserted. The `resistance coil may have a terminal fbar or rod 'il ofrelatively low electrical resistance and greater cross section than the wire at each end, Iandthis terminal member may extend vinto V4the sheath for any desired distance. A simple manner of `attaching the resistance coil to the inner end of the terminal member is to provide such member with a screwthread 5 for adistance of, "say, about 1%. The end of the resistance coil is then :screwed onto the threaded terminal member, a tight t being required. 'While it is old to sofx a resistance coil to a terminal, this typeof engagement could not heretofore be employed in a tube element with insulation formed in situ VVfrom magnesium metal, when the usual Ymethod of assembling all the parts before treatment 'in the autoclave was followed.

"it will be understood, however, that any suitable method of xing the resistance coil to the terminal member maybe followed. Thus, where the resistance wire l'is extremely heavy and it would be' impractical `to 'screw it onto the terminal, .a small hole (slightly larger than the diameter of theresistance wire) may be drilled longitudinally in the end of the Vterminal member to a depth of about the straightened end of the 'resistance coil inserted in the hole, and the end of the terminal member pressed square to clinch the wire -in the hole.

r:Che outside diameter of the resistance coil should be made about two or three thousandths of an inch smaller than the bore of the magnesium hydroxide lining. The vresistance coil, with its terminal members attached, is passed into the central bore and asuitableclip at each end of the sheath-employed to hold the coil and terminal members in place while a refractory insulationpowder, indicatedat l, is poured into the tubefto ll thecentralopening.in the resistance coil as well as the slight clearance space between the terminal members and the magnesium hydroxide lining. When the tube has been lled with the insulation powder to within about y2 from its ends, the terminal members will remain in place while a suitable non-hygroscopic insulation powder 8 is packed around the ends of the terminal members to hold them rmly in place.

The element is nally dried at high temperature to convert the magnesium hydroxide into magnesium oxide, and subjected to compression to compact the assembly. Such compacting of the assembly lprevents .cracks inthe magnesium oxide insulation and insures uniform distribution of the insulation about the coil and terminal member. Because of the uniform density of 'the material ofthe vliner produced by the present method the crystal particles are free to move uniformly under the compacting stress and even sharp turns Y'ofihe "element may be made without disturbingthe continuity of the insulation.

While `the terminal members may be formed from any suitable material, a nickel-chromiumiron alloy, which will not oxidize at the high temperatures at which the element is dried, is to be preferred.

*It will be understood that theoutside tube or sheath, containing the magnesiumtube, may be treated in any length up to the full length of the available autoclave, and the vtube vthereafter cut up into vany lengths required for the heating elements inlquestion.

Manufacturing of the'elements is thus greatly simplified and labour costsin producing tubular elements by the methodof thepresent invention are generally not greater than half those of the usual method as previously outlined.

I claim:

1. A method of making tubular electric heating elements'which' comprises inserting a magnesium metal liner into an outer tubular sheath, hydrating the metal ofthe liner while maintaining the bore of the liner Afree `from obstruction toproduce an expanded lining of magnesium hydroxide having acyiin'olrical bore thereinv of substantially uniform cross-section, and thereafter inserting a resistance coil into the bore of the resultant magnesium hydroxide lining.

.2. A method of making tubular electric heating elements which comprises inserting .a lining of magnesium metal into an outer tubular sheath, hydrating vthe metal of the lining to form Yan expanded magnesium hydroxidelining of uniform density and having a cylindrical bore therein of substantially uniform cross-section, and thereafter inserting a resistance coil having terminal members attached thereto into the bore of the resultant magnesium hydroxide lining with each said terminal Amember having a portion thereof extending within Ysaid cylindrical bore.

./-i method of making tubular electric heating elements which'comprises inserting a substantially seamless lining of magnesium metal into an outer tubular sheath, hydrating the metal of the lining to form an expanded magnesium hydroxide lining A3f-uniform density and having a cylindrical -bcre therein of Ysubstantially uniform cross-section, inserting a'resistance coil having terminal'members'attached'thereto into the bore of the resultant magnesium hydroxide lining with each said terminal member having -a portion thereof extending within said cylindrical bore, filling the central'opening of the resistance coil and the clearance between the terminal members and the lining with refractory powder, drying the assembly to convert the magnesium hydroxide to magnesium oxide, and swaging the sheath to compact and uniformly distribute the insulation around the resistance coil.

4. A method of making tubular electric heating elements which comprises inserting a tubular magnesium substantially seamless liner into a tubular sheath, subjecting the assembly to steam at a pressure of substantially 200 to 500 pounds per square inch and at the corresponding temperature for saturated steam for a period not substantially exceeding four hours to hydrate the magnesium and form an expanded liner of uniform density and having a cylindrical bore therein of substantially uniform cross-section throughf out its length and thereafter inserting an electric resistance element into said cylindrical bore.

5. A method as defined in claim 4 wherein a terminal rod is attached to the resistance element before the latter is inserted in the liner, said terminal rod having a portion thereof extending within said cylindrical bore.

6. A method of making tubular electric heating elements which comprises inserting a substantially seamless lining of magnesium metal into an outer tubular sheath, hydrating the metal oi the lining while maintaining the bore of the lining free from obstruction, thereafter inserting a resistance coil having terminal members atpowder around the terminal members at both ends of the sheath to hold them in place, drying the assembly to convert the magnesium hydroxide into magnesium oxide, and swaging the sheath to uniformly compact the insulation varound the resistance coil.

CHRISTIAN B. BACKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNTTED STATES PATENTS Number Name Date 1,463,005 Dalton July 24, 1923 1,857,615 Backer May 10, 1932 2,219,523 Lampe et al. Oct. 29, 1940 FOREIGN PATENTS Number Country Date 336,949 Great Britain Oct. 22, 1930 

